Hybrid electric vehicles (HEV) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. One method of improving the fuel economy in an HEV is to shutdown the engine during times that the engine operates inefficiently, and is not otherwise needed to propel the vehicle. In these situations, the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or in other cases such as when the battery state of charge (SOC) drops below a certain level, the engine must start quickly and smoothly in a manner that is nearly transparent to the driver.
Starting an engine in an HEV can occur when the vehicle is operating under any one of a number of different conditions. A control strategy for an engine start may be different based on sets of conditions under which the vehicle is operating. The strategy to starting an engine in a HEV may cause the powertrain system to start rough, start sluggishly, cause additional noise and vibration to the powertrain, and/or cause hardware damage to powertrain components. Starting the engine in an HEV when it is moving under the power of the electric motor may cause a noticeable and therefore undesirable torque disturbance in the vehicle driveline. For the hybrid powertrain described above, engine start can be accomplished using different methods, which is the subject of this disclosure.